In fiber optic communications, nonlinearity is a behavior of an optical signal between a transmitter and a receiver in which the optical signal received at the receiver deviates from its normal course and does not vary in direct proportion to the optical signal transmitted at the transmitter. In the art, this is known as perturbation. Examples of nonlinearities include intrachannel nonlinearities, stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), four wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and intermodulation. Nonlinear mitigation techniques are used to simulate the normal course of the optical signal and are key to correcting for perturbation and thereby improving capacity and performance in future optical communications.
Back propagation is a technique that may be used to mitigate nonlinear effects in fiber optic links in fiber optic communications. Back propagation simulates error backward down the fiber optic link to address the non-linear effects within the received optical signal. However, back propagation is very computationally intensive.
Optical nonlinear mitigation using perturbative pre- or post-distortion may be used to increase the reach of optical signals in fiber optic communications. The perturbation induced by the fiber optic link is typically calculated using non-linear perturbation distortion coefficients (Cm,n) and the known or recovered transmitted data. The non-linear perturbation coefficients are typically calculated using known fiber optic link parameters, such as pulse width, inverse of symbol rate, the transmission distance, dispersion map, fiber type, amplifier spacing, and fiber nonlinear parameters.
Optical nonlinear mitigation techniques using perturbative pre- or post-distortion algorithms have been shown to be as effective in mitigating optical nonlinear effects in a fiber link as digital back propagation. Given the non-linear perturbation distortion coefficients (Cm,n), the perturbative pre-distortion algorithm is computationally simpler than back propagation.
However, determining and/or measuring fiber optic link parameters used in the current systems to calculate the non-linear perturbation distortion requires human intervention which may be time consuming and expensive. Additionally, the results of the determinations and measurements may be inaccurate. Further, any change in the fiber optic link parameters requires additional determinations and measurements with the current systems. Therefore, methods and systems are needed to determine non-linear perturbation distortion coefficients automatically at the optical receiver, and ideally without knowledge and/or use of the fiber link parameters.